The present invention relates generally to vehicle drive systems, and more particularly, to a control system for a variable displacement pump useable in a vehicle propel system, such as an auxiliary front wheel drive system.
On a typical motor grader, the rear wheels are driven mechanically by the engine, through a mechanical transmission and differential gear set. It has become well known in the art to supplement such a main, mechanical drive system by means of a hydrostatic, auxiliary front wheel drive system, which is also sometimes referred to as a "supplemental" drive system. Although the pump of the present invention may be utilized on a vehicle having mechanically driven front wheels and hydrostatically driven rear wheels, it is especially advantageous when used on vehicles such as motor graders which typically have mechanically driven rear wheels and hydrostatically driven front wheels, and will be described in connection therewith.
Specifically, the auxiliary drive system includes a fluid pump driven by the engine for providing pressurized fluid to a pair of hydraulic motors, each of which drives one of the front wheels, thereby supplementing or assisting the main, rear wheel drive system. Typically, the fluid pump comprises a variable displacement pump, such as an axial piston pump of the swashplate type. In such a pump, the displacement of the pump is controlled by a pair of stroking cylinders which are actuated by control pressure from a charge pump, with the communication of control pressure to a particular stroking cylinder being controlled by a servo system.
It is generally well known to those skilled in the art to provide an input to the servo system to control pump displacement, in response to various operating parameters of the vehicle drive system, such parameters typically including the speed of rotation of the main propel wheels, and the flow volume of the hydraulic wheel motors. Although such systems have typically controlled front wheel speed, attempts to control wheel torque have typically exacerbated an undesirable phenomenon referred to as "wheel hop", in which the front axle "hops" up and down as the front wheels alternate between an underspeed condition and an overspeed condition, relative to the rear propel wheels.
One of the problems with drive systems of the type described above is that a pressure compensated pump's output pressure is affected by hydraulic leakage, with the amount of leakage being approximately proportional to hydraulic pressure. To reduce the effect of hydraulic leakage on pump output pressure, pump displacement (and flow) is increased by an amount proportional to the product of pump pressure and a leakage gain. Prior to the present invention, a constant was used for the leakage gain, although, as is known to those skilled in the art, leakage gain is actually variable from pump to pump because of variations in part tolerances, operating temperature, and fluid viscosity.